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People who die in their sleep may stop breathing because they have lost too many brain cells, US researchers say.

Sleep apnoea, a condition in which people stop breathing for long stretches of time in their sleep, may sometimes be caused by the destruction of cells in the brain stem, where autonomic functions such as breathing are controlled.

Tests on rats, published online in the journal Nature Neuroscience, show that the loss of key brain stem cells that die off with age cause such disrupted sleep that the animals eventually stopped breathing completely.

The same thing may be happening in elderly people, says neurobiologist Professor Jack Feldman of the University of California, Los Angeles.

"We wanted to reveal the mechanism behind central sleep apnoea, which most commonly affects people after age 65," Feldman says.

"Unlike obstructive sleep apnoea, in which a person stops breathing when their airway collapses, central sleep apnoea is triggered by something going awry in the brain's breathing centre."

Commanding the brain

Feldman and colleagues say they deliberately killed brain cells in the pre-Boetzinger complex of the brains of rats, a region believed to be the "command post" for breathing in mammals.

Then they monitored the rats' breathing.

"We were surprised to see that breathing completely stopped when the rat entered REM [rapid eye movement] sleep, forcing the rat to wake up in order to start breathing again," says Dr Leanne McKay, who worked on the study.

"Over time, the breathing lapses increased in severity, spreading into non-REM sleep and eventually occurring when the rats were awake, as well."

Feldman believes the same thing could be happening in elderly people, especially those with degenerative diseases such as Parkinson's, which are marked by disturbed sleep.

Dying neurones

"Our research suggests that the pre-Boetzinger complex contains a fixed number of neurones that we lose as we age," Feldman says.

"We speculate that our brains can compensate for up to a 60% loss of pre-Boetzinger cells, but the cumulative deficit of these brain cells eventually disrupts our breathing during sleep.

"There's no biological reason for the body to maintain these cells beyond the average lifespan, and so they do not replenish as we age," says Feldman.

"As we lose them, we grow more prone to central sleep apnoea."

And weaker people may not be able to rouse themselves when this happens. They simply stop breathing.